Custom WordPress Slider Shortcode

slider_shortcode.php

<?php
add_shortcode( 'wp_doin_slider', 'wp_doin_slider_func' );

/**
 * @hook slider_test
 */
function wp_doin_slider_func( $atts, $content = null ) {
	$atts = shortcode_atts(
	 array(
	  'ids' => '1,2,3,4',
	 ), $atts, 'bartag' );
        
	// in here, we fetch the ids provided by the user and create an array out of theme to be later used
	// each id corresponds to the id of an image uploaded to the media library
	$image_ids = explode( ',', $atts['ids'] );

	if ( !empty( $image_ids ) ):
		ob_start();
		?>
		<div class="slider__test">
			<?php 
			$counter = 1;
			foreach ( $image_ids as $id ):
				?>	
					<div class="slide slide--<?php echo $counter; ?> slide--active">
						<?php echo wp_get_attachment_image( $id, 'medium' ); ?>
					</div>
				<?php
				$counter++;
			endforeach;
			?>
		</div>
	<?php
	endif;
	return ob_get_clean();
}